EP1618079A1 - Use of a silicon carbide-based ceramic material in aggressive environments - Google Patents

Abstract

A SiC-based composite material capable of use as an inner coating for an aluminium smelting furnace or as an inner coating for a fused salt electrolytic cell, wherein said composite material has been prepared from a precursor mixture comprising at least one beta-SiC precursor and at least one carbonated resin, and wherein said composite material contains inclusions, and wherein at least one part thereof comprises alpha-SiC, in a beta-SiC matrix.

Description

THE USE OF A CERAMIC MATERIAL BASED ON SILICON CARBIDE IN THE

AGGRESSIVE MEDIA

Technical field of the invention

The present invention relates to ceramic materials for use in aggressive media, such as they appear in particular in chemical and electrometallurgical engineering, and more particularly refractory bricks used in melting furnaces or electrolysis tanks.

State of the art

15

Liquid metals and molten salts are among the most aggressive chemical agents known. Many metallurgical and electrometallurgical industrial processes involve the fusion of metals and / or salts, there is a need for refractory materials which resist such an environment. The equipments

20 for molten metals or molten salts, typically melting furnaces or molten salt electrolysis cells, require as internal coating large quantities of refractory plates or bricks, and the replacement of said refractory parts, also called relining, immobilizes the equipment for a certain period of time. A material which has an improved lifespan in such an environment can therefore

25 lead to at least three advantages: 1) less consumption of refractory material, 2) less contamination of the molten medium by said refractory medium, and 3) a downtime of the equipment for maintenance (“down-time”) ) scaled down. In addition, these materials must withstand thermal and mechanical shocks

30 likely to occur when used in such equipment.

The silicon carbide of structure α, which crystallizes in a hexagonal system, linked to other inorganic stabilizers, is one of the materials most used in various industries as ceramic coating thanks to its mechanical properties and exceptional thermal properties, and thanks to its good chemical resistance to corrosive agents, essentially alkalis. Α-SiC refractory bricks are often required for use in molten salt electrolysis cells with an open porosity as low as possible, in order to minimize the penetration of the corrosive medium into the refractory material (see US Patent 5 560 809 (Saint-Gobain)).

This material can be obtained in various macroscopic forms such as brick, cylinder or monolith, chosen according to the intended application. The shaping of silicon carbide generally involves powders or aggregates based on silicon carbide on one side and inorganic binders on the other side. For example, to manufacture refractory bricks for electrometallurgical electrolytic cells, aluminum oxide in solid solution in Si ₃ N ₄ is used as binder, which corresponds to the formula Si _3-x Al _x O _x N _4-x , either silicon oxynitride Si ₂ ON ₂ , or silicon nitride Si ₃ N. The shaping is then carried out by a process making it possible to intimately bond the compounds constituting the final composite, such as hot sintering.

This composite material has an interesting combination of physical and chemical properties, such as high mechanical strength (in particular breaking strength and hardness), high thermal resistance (in particular a low coefficient of expansion and high thermal stability), and a high resistance to oxidation, thanks to which the material can be used in the open air at temperatures exceeding 1000 ° C. This material also resists weakly alkaline solutions.

It also suffers from certain disadvantages. Its relatively high cost is linked to the use of high temperatures during the synthesis. With regard to its resistance in a corrosive medium, there is observed for certain types of binders embrittlement with respect to certain highly corrosive chemical agents initially present in the medium of use, or formed in the medium of use. It is observed in fact that during certain applications of this material, the presence of corrosive products, such as acids or fluorinated compounds or strongly basic products, leads to a progressive destruction of the constituent compounds of the binder. This ultimately leads to the total dissolution of said binder, and consequently to the destruction of the macroscopic form of the material; thus the α-SiC part, for example a refractory brick, turns into powder and / or aggregates, losing its shape and its initial mechanical characteristics.

Two routes have been explored in the prior art for protecting SiC-based composites against corrosion: covering ceramic parts based on SiC with a protective layer which is more resistant to corrosion, and the use of more resistant binders. to corrosion ..

The first route is represented by patent application FR 2 806 406 Al (Commissariat à l'Energie Atomique). It describes a process for depositing a layer on the surface of SiC-based composites, such as pressureless sintered SiC, Si-infiltrated SiC, porous recrystallized SiC, in order to protect them against corrosion and increase their chemical resistance when used at a temperature up to 1600 ° C. The process used consists in preparing a mixture by dispersing in a liquid organic binder the various constituents of the deposition layer, ie metal or silicide, Si, SiC and / or carbon, then covering the surface of the part to be protected by said mixture. The assembly is then heated to a temperature between 1200 and 1850 ° C. The surface of the piece of SiC to be treated is covered by melting said mixture on the surface of said piece to be treated. The deposit thus formed has an average thickness ranging from 1 to 50 μm. The nature and resistance of the deposit varies depending on the nature of said metal and the composition of the applied layer. Nevertheless, the examples given relate only to compositions of the deposition layer and the observation of the latter by electron microscopy without there being any more information on the increased resistance of said composite vis-à-vis the oxidation or aggressive media which constitute the desired end goal.

A specific drawback of this route is the possible appearance of microcracks between the protective layer and the composite during the manufacture or use of parts thus coated, due to differences between the coefficients of thermal expansion.

The second path is based on the idea that it is the chemical nature of the binders used in SiC-based ceramic materials which determines their resistance to attack by aggressive chemical agents such as fluorinated derivatives or concentrated acids. or alkalis. According to the state of the art, binders based mainly on sinterable oxides, nitrides or oxynitrides are used.

In the known processes, silicon carbide of structure α is used in the form of a powder resulting directly from syntheses by traditional carboreduction according to reaction (i):

SiO ₂ + 3 C → α-SiC (or β-SiC) + 2 CO (1) Reaction (1) can be broken down into two elementary reactions which are as follows: SiO ₂ + C → SiO + CO (2)

SiO + 2 C → SiC + CO (3)

However, the SiO escapes very quickly from the reaction medium before the reaction (5) is complete and therefore causes a significant loss of the starting silicon, leaving a large amount of unreacted carbon in the final material. US Patent 4,368,181 (Hiroshige Suzuki) proposes to improve this process by reacting fine particles of carbon, ie average diameter of the order of 60 μm, and of silica, ie average diameter of the order of 150 μm, in a device for continuous recycling in order to reduce the loss of silicon by loss of SiO and to increase the yield of SiC. However, the SiC thus formed by this process is still in the form of a very fine powder and requires another preforming step with binders before use. These binders are liable to be corroded by strongly acidic or basic solutions, with the result that the macroscopic structure of the material is destroyed.

Furthermore, there is known from patent EP 511 919 a method of manufacturing porous SiC catalyst supports in the form of rods or granules extruded from a mixture of silicon powder and organic resin by polymerization followed by carburetion.

Patent applications or patents US 5,474,587 (Forschungszentrum Julich GmbH), US 2002/011683 Al (Corning Inc), EP 0 356 800 A (Shinetsu Chemical Co), US 4 455 385 (General Electric Co), US 4 562 040 (Sumitomo), US 4 514 346 (Kernforschungsanlage Jϋlich), US 6,245,424 (Saint-Gobain Industrial Ceramics) and US 3,205,043 (The Carborundum Company) also illustrate the manufacture and use of such SiC materials.

Problem

The present invention seeks to overcome these drawbacks of the methods according to the prior art. It aims to offer interior coating products for industrial ovens and electrolysis tanks made of ceramic material based on silicon carbide, which have better resistance to attack by corrosive media, especially fluorinated media, concentrated acids or alkaline media. , while retaining the exceptional physical properties known to SiC.

Description of the figures

Figure 1 shows scanning electron microscopy images of the composite after dynamic vacuum carburetion at 1300 ° C for 2 hours. The wetting of the α-SiC grains by the β-SiC matrix is visible on the microscopy image presented in Figure 1B.

FIG. 2 shows optical images of a composite consisting of aggregates based on α-SiC with binders based on Al ₂ O ₃ and Si ₃ N before (A) and after (B) quenching in a 40% HF solution by volume for 10 hours. The dissolution of the binders by the HF resulted in the total destruction of the macroscopic structure of the starting material.

The images (C, D) correspond to a composite constituted by aggregates of α-SiC in a matrix of β-SiC having undergone the same treatment as that of images (A) and (B). We appreciate the strong resistance of this material to attacks by highly corrosive solutions. Subject of the invention

The subject of the present invention is the use of a SiC-based composite material as an interior coating for an aluminum smelting furnace or as an interior coating for an electrolysis cell in molten salt, characterized in that said composite material contains inclusions, at least part of which is α-SiC, in a matrix of β-SiC.

Description of the invention

The problem is solved according to the present invention by replacing the oxide-based binders used in the known processes with a matrix constituted by silicon carbide of structure β (which crystallizes in a face-centered cubic system), and by l 'addition of inclusions

Such a material can advantageously be produced by a process which comprises

(a) the preparation of a mixture known as a “precursor mixture” comprising at least one β-SiC precursor, which can be in particular in the form of powder, grains, or fibers of various sizes, with at least one carbonaceous resin, preferably thermosetting,

(b) the shaping of said precursor mixture, in particular in plates or bricks;

(c) polymerization of the resin, (d) heat treatment at a temperature between 1100 and 1500 ° C to remove the organic constituents from the resin and form the final part.

The term “β-SiC precursor” is used to designate a compound which forms, under the conditions of the heat treatment (step (d)), the constituents of the β-SiC resin. As precursor of β-SiC, silicon is preferred, and more particularly in the form of a powder. This silicon powder can be a commercial powder, of known particle size and purity. For reasons of homogeneity, the particle size of the silicon is preferably between 0.1 and 20 μm, preferably between 2 and 20 μm and more especially between 5 and 20 μm.

Any resin containing carbon atoms is called a "carbon resin". It is neither necessary nor useful for it to contain silicon atoms. Advantageously, the silicon is provided only by the precursor of β-SiC. The resin is advantageously selected from thermosetting resins containing carbon, and in particular from phenolic, acrylic or furfuryl resins. A phenolic type resin is preferred.

In the precursor mixture, the respective amounts of resin and β-SiC precursor are adjusted so as to transform the β-SiC precursor quantitatively into β-SiC. To this end, the quantity of carbon contained in the resin is calculated. Part of the carbon can also be provided by direct addition of a carbon powder into the mixture of carbon resin and the precursor of β-SiC. This carbon powder can be a commercial powder, for example carbon black, of known particle size and purity. For reasons of homogeneity of the mixture, a particle size of less than 50 μm is preferred. The choice of the composition of the mixture results from a compromise between the viscosity, the cost of the raw materials and the desired final porosity. To ensure the complete transformation of the precursor of β-SiC into β-SiC and thus allow the production of a final material free of Si not engaged in the structure of SiC, a slight excess of carbon is preferred in the precursor mixture. This excess carbon can then be burned in air. However, the excess carbon should not be too high so as not to generate too large a porosity inside the material after combustion of the residual carbon, thus inducing embrittlement in the mechanical strength of the final composite. It is possible to carry out a second infiltration of the composite thus synthesized by the resin / Si mixture, in order to reduce the porosity at the heart of the composite. This is useful for certain applications which imperatively require minimizing the porosity.

The precursor mixture can be shaped by any known process such as molding, pressing, extrusion to obtain three-dimensional shapes such as bricks, plaques, or tiles. The method chosen will be adapted to the viscosity of the precursor mixture, itself a function of the viscosity of the resin and of the composition of the precursor mixture. By way of example, it is possible to obtain, for example, plates with a thickness of 1 mm and a length and width of one to several decimeters. You can also make bricks with dimensions from a few centimeters to a few decimeters or more. Parts of more complex shapes can also be obtained, in particular by molding.

Said precursor mixture is then heated in air to a temperature between 100 ° C and 300 ° C, preferably between 150 ° C and 300 ° C, more preferably between 150 ° C and 250 ° C, and even more preferably between 150 ° C and 210 ° C. The duration of this treatment, during which the polymerization of the resin takes place and the hardening of the part, is typically between 0.5 hours and 10 hours at the temperature level, preferably between 1 h and 5 h, and even more preferably between 2 and 3 hours. During this stage, the material gives off volatile organic compounds which create a variable residual porosity as a function of the level of carbon present in the composition of the precursor mixture and of the conditions applied during the polymerization. It is preferable to minimize this porosity, especially for the manufacture of thick plates (typically thickness of at least 2 mm) and bricks. An intermediate part is thus obtained which has a certain mechanical strength and which can therefore be easily handled.

Said intermediate part thus obtained is subjected to heating under an inert atmosphere (for example helium or argon) or under dynamic vacuum between 1100 ° C. and 1500 ° C. for a period ranging from 1 to 10 hours, preferably between 1 and 5 hours and more especially between 1 and 3 hours in order to carry out the carbonization of the resin then the carburetion reaction of the matrix. The optimum temperature range is preferably between 1200 ° C and 1500 ° C, more especially between 1250 ° C and 1450 ° C. The most preferred range is between 1250 ° C and 1400 ° C. The SiC formed from the carbon originating from the resin and from the precursor of β-SiC is β-SiC. When the carburetion treatment is carried out under inert gas, the presence of traces of oxygen is preferable, especially when the resin has an excess of carbon. In this case, the carburetion can be carried out for example under an atmosphere containing traces of oxygen. In some cases, oxygen from impurities in commercial argon may suffice. In the case where the product after carburetion has a high residual carbon content, this can be easily removed by heating the part in air at a temperature between 600 ° C and 900 ° C, preferably between 700 ° C and 825 ° C, for a period advantageously between 10 minutes and 5 hours.

The Applicant has found that the rate of polymerization influences the residual porosity in the final material, since too rapid polymerization promotes the formation of gas bubbles. However, the presence of gas bubbles in the resin can promote the formation of microcracks in the ceramic composite, which can weaken the material part during its use. This problem can arise in particular when manufacturing plates with a thickness of at least 1 mm, and bricks. It is therefore useful to carry out the polymerization in a fairly slow manner, that is to say at a moderate temperature.

For the first infiltration step, the preferred method involves a carbon resin, but does not require the use of an organic silicon-based resin, such as polycarboxysilane or polymethylsilane, which are used in known methods of manufacture of ceramics incorporating SiC fibers; see EP 1 063 210 A1 (Ishikawajima-Harima Heavy Industries, Ltd.); these organic silicon-based resins are relatively expensive, and there is a significant loss of carbon after pyrolysis.

The process described above allows the manufacture of bricks or refractory plates based on β-SiC without inclusions. When no inclusions are added (for example in the form of α-SiC), said refractory bricks or plates have a density typically of the order of 1.5 g / cm ³ . This value is too low for certain use in a corrosive environment, and in particular in a fluorinated environment. In an advantageous embodiment, plates with a thickness of at least 1 mm, preferably at least 3 mm, and even more preferably at least 5 mm are manufactured. The smallest section of said plates is advantageously at least 15 mm, and preferably at least 50 mm, with a ratio of length or width to thickness of at least 10 and preferably at least 15. In another mode advantageous realization, bricks are made. The smallest dimension of said bricks is advantageously at least 10 mm, and preferably at least 50 mm or even 100 mm. The smallest section of said bricks is advantageously at least 20 cm ² , preferably at least 75 cm ² and even more advantageously at least 150 cm, with a ratio of length or width to thickness of at least 3.

In both cases, it is advisable to limit the excess of carbon and to polymerize slowly to avoid the formation of large bubbles capable of weakening the material during its carburetion. For the use of the material as lining for an industrial oven, the material is prepared especially in the form of plates or bricks, which may have the shape of a parallelepiped or any other suitable shape.

The Applicant has found that for the use of the material in industrial ovens or electrolysis tanks, it is particularly advantageous to add to the precursor mixture inclusions of which at least a part consists of α-SiC. In this case, step (a) indicated above is replaced by step (aa):

(aa) the preparation of a precursor mixture comprising inclusions, at least part of which consists of α-SiC, and a precursor of β-SiC, which may be in the form of powder, grains, fibers or d '' inclusions of various sizes, with a carbon resin, preferably thermosetting.

Typically, α-SiC of variable particle size ranging from 0.1 to several millimeters is used as inclusions. This alpha-form silicon carbide can consist of any of the silicon carbides known to date. The inclusions are added to the precursor mixture in a proportion of at least 80% (by weight relative to the total mass of the precursor mixture). Below 80%, the density of the finished part is too low, its open porosity is too high, and the green part (part formed before cooking) is too soft. Above 95%, the β-SiC binder can no longer completely wet the inclusions, which results in insufficient cohesion of the finished part. A fraction of approximately 90% inclusions is well suited for most applications in a corrosive fluorinated medium.

Part of the α-SiC can be replaced by alumina, silica, TiN, Si ₃ N or other inorganic solids which do not decompose and do not sublime at the synthesis temperature of the final composite. Advantageously, at least 50% and preferably at least 70% by weight of the inclusions consist of α-SiC. According to the Applicant's observations, for the use of the material as an interior coating for aluminum electrolysis cells or as an interior coating for an aluminum melting furnace, the substitution of α-SiC by other inorganic inclusions does not provide any significant technical advantage.

The solid constituting the inclusions is not limited to a precise macroscopic form but can be used in different forms such as powder, grains, fibers. By way of example, to improve the mechanical properties of the final composite, the fibers based on α-SiC are preferred as inclusions. These fibers can have a length that exceeds 100 μm.

These inclusions, at least part of which must consist of α-SiC, are mixed with a carbonaceous resin, preferably thermosetting, containing a given amount of a precursor of β-SiC, preferably in the form of powder with a particle size ranging from 0.1 to several micrometers. A composite material of the α-SiC / β-SiC type is thus obtained, comprising particles of α-SiC in a matrix of β-SiC, which does not need to contain other binders or additives.

A second infiltration treatment can be carried out according to the same procedure described: soaking of said material in a mold containing the resin, polymerization and finally, carburetion treatment. Said resin must contain a sufficient amount of the β-SiC precursor, for example in the form of silicon powder. This second treatment improves mechanical strength and / or eliminates the problems inherent in the presence of an undesirable porosity better resistance to attack by corrosive media, especially fluorinated media, concentrated acids or alkaline media.

The heat treatment is also simplified because the composite can be formed indifferently either under dynamic vacuum or under inert atmosphere, ie argon, helium without there being any need for precise control of the purity of said atmosphere, ie trace of oxygen or water vapor present as impurities in the gas used. In addition, the carburetion reaction proceeds by nucleation inside the carbon / silicon matrix and therefore is completely independent of the size of the composite to be manufactured.

In a preferred variant of the process, the carbon and the silicon are intimately mixed in the following manner: the silicon powder (average grain size of approximately 10 μm), is mixed with a phenolic resin which, after polymerization, provides the source of carbon necessary for the reaction for the formation of β-SiC. The inclusions are then mixed with the resin and the whole is poured into a mold having the shape of the desired final composite. After polymerization, the solid formed is transferred to an oven allowing the final carburetion of the matrix to be carried out. During the temperature rise, the oxygen of structure or trapped in the matrix reacts with silicon and carbon to form SiO (equation (4)) and CO (equation (5)) inside the matrix solid. Carburation then proceeds by reaction between SiO and carbon (6) or CO with Si (7) according to the following equations:

2 Si + O ₂ → 2 SiO (4)

2 C + O ₂ → 2 CO (5) SiO + 2 C → SiC + CO (6)

2 CO + 2 Si → 2 SiC + O ₂ (7) The fact that all the components are intimately mixed considerably increases the final yield of SiC with very little loss of silicon in the gas phase. The synthesis method also makes it possible to manufacture SiC with a predefined macroscopic form and not in the form of a fine powder as was the case with the results of the prior art.

The process described above makes it possible to produce materials or composites with a matrix based on β-SiC which may contain inside inclusions based on silicon carbide or other materials resistant to use in aggressive, highly acidic or basic, and under strong temperature constraints.

The composite material based on SiC which contains in a matrix of β-SiC inclusions of which at least a part in α-SiC, has many advantages:

(i) It can be manufactured by the process described above with a relatively low cost price compared to other processes, taking into account the costs of raw materials (resin constituting the carbon source, silicon powder) and thanks a significant energy saving, the process involving relatively low temperatures, ie <1400 ° C. The limited number of raw materials also allows a significant reduction in cost. (ii) The shaping of the mixture can be carried out preferably before the polymerization by extrusion, molding or pressing. It is easy taking into account the nature of the starting material, namely a viscous matrix based on resin, silicon powder and inclusions in the form of powder and / or grains of dispersed α-SiC. This allows the material to be preformed into relatively complex shapes. Alternatively, the part can be shaped by machining after polymerization of the resin, preferably before the heat treatment (step (d)).

(iii) The strong chemical and physical affinity between the various constituents of the composite allows better wetting of the grains or inclusions of α-SiC by the matrix based on β-SiC. This is due to their close chemical and physical natures despite their different crystallographic structure, ie α-SiC (hexagonal) and β-SiC (cubic). These similarities come essentially from the specificity of the chemical bond Si-C which governs most of the mechanical and thermal properties as well as the high resistance to corrosive agents. They also allow the creation of strong bonds between the two phases (β-SiC matrix and inclusions) avoiding the problems of rejection or delamination during use under stress. In addition, the α-SiC inclusions have a coefficient of thermal expansion very close to that of the β-SiC matrix, making it possible to avoid the formation of residual stresses which may appear within the composite during the heat treatment or during the cooling; this avoids the formation of cracks which could be damaging to the finished part, in particular when it is used in aluminum melting furnaces or in molten salt electrolysis cells, and which can be difficult to detect on the finished part.

(iv) The Applicant has found that the composite material described has an extremely high resistance to corrosive media, in particular to fluorinated media, to concentrated acids or to alkaline media. This is probably due to the absence of binders having a lower resistance to said corrosive media. Parts made of this material or composite therefore allow better economy of use. More particularly, in a given aggressive medium, the service life of the parts made of composite material is greater than that of the parts based on SiC using binders which do not resist well to these aggressive mediums. This also improves the safety of use of the SiC parts, in particular their sealing, and opens other applications impossible to envisage with materials based on SiC whose binders are not chemically inert.

(v) By varying the chemical and physical nature of the inclusions, the process described also makes it possible to prepare other types of composite containing not only only silicon carbide but other materials such as alumina, silica or all other compounds, provided that they can be dispersed in the resin and that they are not altered during synthesis. The addition of these inclusions other than α-SiC, in a variable proportion, makes it possible to modify the mechanical and thermal properties of the final composite, ie improvement of thermal transfer, resistance to oxidation or clogging of pores. The material can thus be adapted to the precise requirements of the intended use. (vi) By varying the proportion of inclusions, and in particular the mass percentage of α-SiC, the thermal and mechanical resistance of the material can be varied, depending on the intended application.

The Applicant has found that this SiC-based material containing inclusions, at least part of which is α-SiC, in a matrix of β-SiC, can be used, in particular in the form of refractory bricks, as a material for coating in various applications relating to thermal engineering, chemical engineering and / or electrometallurgical engineering which must respond to high mechanical and thermal stresses, and / or in the presence of corrosive liquids or gases. It can be used in particular in constituent elements of heat exchangers, burners, ovens, reactors, or heating resistors, in particular in oxidizing medium at medium or high temperature, or in installations in contact with corrosive chemical agents. It can also be used as a constituent in certain parts used in the fields of aeronautical or space technology and land transport. It can also be used as a material used in the manufacture of utensils used as a crucible support for high temperature applications such as the synthesis of monocrystalline silicon bars. The material can be used as an interior lining for furnaces, such as aluminum melting furnaces, and as a lining for molten salt electrolysis cells, for example for the production of aluminum by electrolysis from a mixture of alumina and cryolite. It can also be used as a component of a heat shield in a spacecraft.

Another use of these materials is that as lining (interior coating) of incineration ovens, such as incineration ovens of household waste. During incineration, corrosive gases (HF, HC1, Cl ₂ , NO, NO ₂ , SO ₂ , SO ₃ , etc.) can be formed; these can attack the interior lining of the oven.

The density of the material described is preferably greater than 2.4 g / cm ³ . For the indicated uses, a density between 2.45 and 2.75 g / cm ³ is particularly suitable. Examples

Example 1: Production of β-SiC plates without inclusions

1500 g of silicon powder (granulometry centered on 7 μm), 560 g of carbon black (granulometry centered on 20 nm) and 1000 g of phenolic resin are mixed in a mixer.

The paste thus obtained is then compressed between two flat surfaces to obtain a plate with a thickness of 3 mm. This plate is cured by heating at 200 ° C for 3 hours. During this step, a weight loss corresponding to approximately 10% of the initial weight of the mixture is observed. The part obtained is easy to handle and has a smooth surface appearance.

Said part is then subjected to a progressive heating under scanning of argon at atmospheric pressure up to 1360 ° C., then it is maintained for one hour at this temperature. The piece is finally allowed to cool to room temperature. During this step, a weight loss corresponding to approximately 13.5% of the hardened part is observed. The appearance of the material is black because it still contains around 7% free carbon. This is then removed by heating in air at 700 ° C for 3 h. The plate then has a gray color characteristic of pure β-SiC. The density of this plate was 1.2 g / cm ³ . It did not show any cracks. By a very similar process, refractory bricks were made of β-SiC with a smaller dimension greater than or equal to 15 cm, without cracks.

Example 2: Production of β-SiC plates with inclusions of α-SiC ( ^" α-SiC composite

/ β-SiQ

Variant (a):

4.5 g of silicon powder (average particle diameter: about 7 μm) are mixed with 5.5 g of a phenolic resin constituting the carbon source necessary for carburetion to form β-SiC intended to play the role of binder in the final composite. 7 g of α-SiC in powder form are added to this mixture as a source of inclusions. The mixture was shaped by molding. The whole is polymerized in air at 150 ° C for 2 h. The loss in mass during this polymerization was 2 grams. The solid thus obtained is subjected to a heat treatment under dynamic vacuum at 1300 ° C. with a temperature rise slope of 5 ° C. min ^"1. During the temperature rise, the polymerized resin chars and leads, at high temperature , to a carbon skeleton in intimate contact with the silicon grains, which facilitates the synthesis of SiC. The composite is maintained at this temperature for 2 h in order to transform the mixture of carbon coming from the carbonized resin and the silicon into β- The composite obtained is then cooled with the natural thermal inertia of the furnace to ambient temperature, the mass loss during this heat treatment step was 1 gram.

The product thus obtained is composed of a mixture of 50% of α-SiC and 50% of β-SiC in which the aggregates of α-SiC are homogeneously dispersed in a matrix based on β-SiC. It has physicochemical properties close to or similar to those of composites based on aggregates of α-SiC dispersed in an alumina and Si ₃ N ₄ matrix. The images obtained by scanning electron microscopy of the composite after polymerization and after carburization are presented in FIG. 1. The low resolution image (FIG. 1A) clearly shows a homogeneous dispersion of the grains of α-SiC through the matrix consisting of β-SiC generated by the reaction at 1300 ° C between the carbon of the resin and the silicon. We also observe the presence of residual porosity in the final composite. This residual porosity probably originates from contractions taking place at the heart of the resin during the polymerization step. This residual porosity can be eliminated by adjusting the heating slope during the polymerization step or by using a different resin. The wetting between the two phases can be better seen by the higher magnification microscopy image presented in Figure IB. This wetting is explained by the very close physicochemical properties of the two materials which inhibit the problems of rejection during heat treatment as was the case with other binders not having the same coefficient of thermal expansion as carbide. silicon to protect. This way of preparing the composites makes it possible to vary over a wide range the mass percentage of starting α-SiC, in order to adapt the properties of the composite, such as its mechanical strength and its porosity, to the intended applications. Variant (b):

In another variant, a mixture of 4.5 g of silicon powder, 5.5 g of phenolic resin and 73 g of α-SiC grains is produced. The mixture is shaped by pressing so that the resin and the silicon powder fill most of the free volume between the grains of α-SiC.

The procedure is then identical to Example 2 (a).

The product obtained then consists of a mixture of 91% of α-SiC linked by 9% of β-

SiC and has a density of 2.5 g / cm ³ with an open porosity of less than 20%.

Example 3: Use of β-SiC / α-SiC composite plates in a corrosive environment

This example makes it possible to better appreciate the extreme resistance of the composite material based on β-SiC with inclusions (see example 2) compared to a composite based on α-SiC with binders based on oxides and / or nitrides. For this purpose, a 40% by volume HF solution was used as the corrosive medium. It is known that vapors or liquids containing hydrofluoric acid are extremely corrosive to binders based on ceramic oxides, causing severe problems of destruction of the matrix. The results are presented in Figure 2.

The composite α-SiC / binders based on oxides and / or nitrides (Figure 2 A) is completely destroyed after treatment in the HF solution resulting in the complete destruction of the matrix and only of the α-SiC powder of departure is retrieved (Figure 2B). The α-SiC / β-SiC composite prepared according to Example 2 (variant (a)) remains stable, and no apparent modification was detected after the treatment in THF (Figures 2C and D). This illustrates the chemical inertia of the matrix based on β-SiC vis-à-vis the HF solution.

By similar tests, the Applicant has found that the composite based on β-

SiC with inclusions also withstands treatments in basic environments such as hot concentrated soda. The composite based on α-SiC / binders based on oxides and / or nitrides is destroyed after a similar treatment, because the concentrated sodium hydroxide dissolves the binders.

This test was repeated with the material from variant (b) of Example 2. The resistance to HF was excellent.

Claims

1. Use of a SiC-based composite material as an interior coating for an aluminum smelting furnace or as an interior coating for a molten salt electrolysis cell or as an interior coating for an incineration furnace, characterized in that said composite material was prepared from a mixture called “precursor mixture” comprising at least one precursor of β-SiC and at least one carbon resin, and in that said composite material contains inclusions, at least part of which α-SiC, in a matrix of β-SiC. 2. Use according to claim 1, wherein the weight fraction of said inclusions is between 80% and 95% relative to the total mass of the precursor mixture. 3. Use according to claim 1 or 2, wherein a part of said inclusions are alumina, silica, TiN, Si ₃ N ₄ or a mixture of these compounds. 4. Use according to any one of claims 1 to 3, in which at least 50% by weight of said inclusions, and preferably at least 70% by weight of said inclusions, are α-SiC. 5. Use according to any one of claims 1 to 4, wherein said material has a density of at least 2.4 g / cm ³ , and preferably a density between 2.45 and 2.75 g / cm ³ . 6. Use according to any one of claims 1 to 5, wherein said material is used in the form of brick or plates. Use according to any one of claims 1 to 6 as lining of an electrolytic cell for the production of aluminum from a mixture of alumina and cryolite.